Added "barrier" to OpenCL syntax. Implemented its event-structure semantics. Provided "same_B" (same barrier id) relation and "entry_fence" and "exit_fence" sets as new .cat primitives.
Merge commit 'f931af575572f2fd9e152fe4bd4073306ca162af'

Conflicts:
	herd/OpenCLAction.ml
Oups.